A New Planet and Its Implications

by Paul Gilster on July 23, 2007

What are the two most fundamental properties of the stars we study? If you said mass and chemical composition, you get the prize, at least as determined by the California & Carnegie Planet Search team. Their new paper lays out the discovery of a gas giant orbiting the M-class red dwarf GJ 317. And they first discuss the discovery in the context of the core accretion model for planetary formation, and the correlation between the metallicity of a star and the chances of its harboring detectable planets.

The notion seems sound: The host star inherits its characteristics from the same disk out of which the planets around it form. If you increase the amount of metals in the system (metals being defined as elements higher than hydrogen and helium), you increase the surface density of solid particulates, and that ought to bump up the growth rate for the core materials that become planets. In a gas giant, such a core then becomes massive enough to capture a gas envelope.

But the case around M dwarfs, those dim red stars that may comprise as much as 75 percent of the galaxy, needs a special look. If the mass of a protoplanetary disk scales with the mass of its central star, then larger mass stars should be more likely to produce planets. Greg Laughlin (UC-Santa Cruz) has studied this relationship in low mass stars like red dwarfs. His finding: Because their disks have lower surface densities (and longer orbital time scales), such stars should have problems producing Jupiter-mass planets. That leads to Neptune mass worlds that have exhausted their supply of gases in the disk through which they move.

So far, the analysis squares with observation. Most of the planets detected around M dwarfs are a good deal smaller than Jupiter. Until the California & Carnegie team’s recent work, only two nearby M dwarfs were known to have Jupiter-mass companions: GJ 876 and GJ 849. The upshot is that the frequency of giant planets is two to three times higher among stars of the Sun’s mass as compared to M dwarfs. All this, of course, has to be kept in the context of the relatively small sample within whose confines we work.

But GJ 317 now adds to the total, with the team finding a gas giant in an 1.897 year orbit around the star. We now have six M dwarfs known to harbor at least one Doppler-detected planet, with GJ 317 being just the third out of 300 surveyed to show a Jupiter-class world. A second possible Jovian planet is also being monitored in a 2700 day orbit, but no firm detection is yet being claimed.

But if the second planet is borne out, the result shouldn’t surprise us. Note this from the discovery paper (internal references deleted for brevity):

Multi–planet systems appear to be relatively common among M dwarfs compared to Sun–like stars. All M stars with one Jovian planet show evidence of a second companion. GJ 876 has a pair of Jupiter–mass planets in a 2:1 mean motion resonance, along with an inner “super Earth” GJ 849 has a long–period Jovian planet with a linear trend. Of the 3 M dwarfs with Neptune–mass planets, two have multiple planets or evidence of an additional companion: GJ 581 harbors 3 low–mass planets, and GJ 436 has a linear trend. Only GJ 674 appears to be in a single–planet system. From the ﬁrst 6 planet detections around low–mass stars, it appears as though M dwarfs have an 80% occurrence rate of multi–planet systems, compared to the 30% rate measured for FGK stars.

Interesting, no? One possible cause for the difference is that fact that planets around low mass stars are more readily detectible by radial velocity methods. On the other hand, the team notes that all the Jovian planets detected around M dwarfs so far would have been detectable around Solar-mass stars. So we seem to be looking at a real effect here, one that will demand the accumulation of a lot more data through future surveys before its implications are fully understood.

Another key finding of this work is that A-type stars in the broader stellar sample studied are fully five times more likely than M dwarfs to harbor a gas giant. The team’s conclusion:

This important result establishes stellar mass as an additional sign post for exoplanets, along with metallicity. Just as metallicity informs the target selection of searches for short–period planets, stellar mass will be an important factor in the target selection of future high–contrast direct imaging surveys. While the lower luminosities of M dwarfs provide favorable contrast ratios that facilitate the detection of thermal emission from young giant planets, our results show that A–type stars are far more likely to harbor such planets.

Thus the correlation between stellar mass and the likelihood of finding giant planets seems to be firming up, but as the authors note, we need a larger star sample to really understand the relationship. The California & Carnegie team have added a number of higher mass stars to their earlier samples in hopes of tightening the focus. As we range between the smallest and some of the largest planet host stars, we’ll not only find more and more planets, but also uncover facts that should help in the target choice for future planet-hunter spacecraft.

The paper is Johnson et al., “A New Planet Around an M Dwarf: Revealing a Correlation Between Exoplanets and Stellar Mass,” accepted by the Astrophysical Journal and available online.

What’s neat about A and F stars is that they would have HUGE habitable zones potentially supporting many planets with liguid water and moderate temperatures. Imagine a solar system with 3 or more Earths. Since the stars are short lived, we’d need to accelerate the life complexity process seeding them with Earth based complex life forms which could thrive for millions or tens of millions of years. Of course there’s the minor problem of geting there…

dad2059 is right about all those ‘hot Jupiters.’ But do bear in mind that one reason we’ve found so many of them so early is that they are among the easiest for our radial-velocity searches to detect. Hence our early planet finds are probably biased in that direction. As the radial-velocity datasets grow larger and larger for individual stars, we should be able to get a read on a much wider range of planetary types.

In the article:
“Multi–planet systems appear to be relatively common among M dwarfs compared to Sun–like stars. All M stars with one Jovian planet show evidence of a second companion.”

in the comments:
“But do bear in mind that one reason we’ve found so many of them so early is that they are among the easiest for our radial-velocity searches to detect.”

Together, these come to my point: variation in radial velocity of the primary is related to the relative masses of the primary and its planets. This implies that small planets are easier to detect around small (M) stars than around bigger (F and G) stars; who knows how many earth-mass stars around larger stars have not been detected because they are below our current detection limit? Similarly, the hot jupiters may be over-represented in the current exoplanet catalog simply because they are easiest to detect. We should be careful about extrapolating these samples to the galaxy at large.

Yes! This is in line with what I mentioned in an earlier post Exoplanet Prediction by Stellar Elements (ref. Lineweaver’s work on this), June 9th:

“I always thought that stellar characteristics would determine planetary system composition, in particular metallicity and stellar mass. ‘Knowing the mother is knowing the children’ “.

From the article itself:

“We find a positive correlation between stellar mass and the occurrence rate of Jovian planets within 2.5 AU; (…). Our analysis shows that the correlation between Jovian planet occurrence and stellar mass remains even after accounting for the effects of stellar metallicity”.

I already noticed a few years ago (OK, as an amateur, looking at data from the Extrasolar Planet Encyclopedia), when the California & Carnegie Planet Search team published about the correllation between stellar metallicity and planet occurrence, that the correllation becomes (a bit) stronger when both metallicity ánd stellar mass together (for instance metallicity * mass, expressed as x times solar) are related to (total) planetary mass.

The correllation was still rather weak though, possibly because not all giant planets had been discovered for those stars yet (i.e. not yet the ones in greater orbits).

(With regard to metallicity: as Lineweaver suggested, the correllation may become clearer and stronger as we learn to distinguish between various elements, in stead of just overall metallicity).

@djlactin: agree, but what may be tentatively possible is what the California & Carnegie Planet Search team did a few years ago: extrapolating the planetary abundance to planetary mass ratio (see their web site exoplanets.org), finding, even despite the bias towards giant planets, that the smaller the planetary mass the more abundant.

Very promising.

@Adam: agree on the short lifetimes of A and F stars, but that is probably why philw speaks about the “need to accelerate the life complexity process seeding them with Earth based complex life forms which could thrive for millions or tens of millions of years”.
Kind of terraforming or at least propagating life on lifeless but potentially habitable planets.

@philw: are the *continuously* habitable zones of A and F stars wide enough for 3 or even 2 habitable planets? In other words: are they really that wide and isn’t it so that they move outward relatively fast?

Ronald, you expressed my A and F terraforming idea well. Yes the stars are short lived and yes, early A star planets would not last long enough to cool down geologically as Adam suggested. But plenty of possibly late A and certainly F stars with luminosities 1.5 to several suns would have habitable zones that would be ‘stable’ for human horizon purposes of terraforming. Were the planets to have oceans and land. Nanotech (i.e. advanced biotek) technology could seed O2 producing organisms, create soil, etc. The hardest part from today’s horizon is the seemingly impossible task of geting there.

And yes, the width of the HZ is a function of the luminosity. Multiple Earth type planets ‘could’ exist within a wide HZ. And yes it moves fast, but not fast on a thousands of years timescale.

@philw: great ideas. But since A and F stars are relatively scarce in the MW galaxy, and short-lived and emitting rather aggressive radiation, I would still prefer to go for the more solar (spectral) type stars: roughly from F5 to K2. And as we are learning now, the smaller, cooler stars probably show a scarcity of planets combined with a very narrow HZ.
Maybe the solar stars really constitute a window of opportunity for (suitable) planets, in several ways: insolation and temperature, lifespan, stability, metallicity (well, at least part of them), mass, HZ, …

Interestingly enough, according to Kasting et al. 1993, “Habitable Zones around Main Sequence Stars”, the continuously habitable zones around A and F stars of a given duration of habitability are narrower when measured on a logarithmic scale than the comparable zones around G stars, due to faster stellar evolution. The reason given for using a log scale is that the planets in our solar system are logarithmically-spaced. A quick check of some 3 and 4 planet extrasolar systems suggests that they too seem to have planets spaced in a logarithmic pattern (if you allow for unfilled gaps which may perhaps host additional low mass planets or asteroid belts: fitting our solar system to a logarithmic spacing requires inclusion of the asteroids), so perhaps, paradoxically enough, it is low mass stars which are more likely to have multiple habitable planets.

@andy:
does this then confirm the ‘window of opportunity’ of the solar type stars (late F – early K), the hotter A and F stars having a too narrow CHZ and the cooler (late K, M) stars having a very narrow HZ anyway.

But is seems hard to fit 2 or more terrestrial planets into the CHZ of a solar type star. E.g. a ‘typical’ G star with a CHZ from 0.9 – 1.2 AU orso (maybe a bit more), if you try to fit more than one planet in, it becomes rather crowded for long-term stable orbits. Or not?

While the HZ of a red dwarf star is narrower in real space, consider that Gliese 581 has two planets (c and d) located close to the habitable zone. Same also goes for Gliese 876 (planets b and c).

Formation of two habitable planets around a G star is probably possible (I’ve seen a paper about terrestrial planet accretion that ends up with such a configuration, both planets just on the edge of the HZ), but the planets would probably interact quite significantly: in the configuration produced by the accretion simulation, the two planets traded orbital eccentricity back and forth.

Very hot Jupiters (VHJs) are defined as Jupiter-mass extrasolar planets with
orbital periods shorter than three days. For low albedos the effective
temperatures of irradiated VHJs can reach 2500-3000 K. Thermal emission from
VHJs is therefore potentially strong at optical wavelengths. We explore the
prospects of detecting optical-wavelength thermal emission during secondary
eclipse with existing ground-based telescopes. We show that OGLE-TR-56b and
OGLE-TR-132b are the best suited candidates for detection, and that the
prospects are highest around z’-band (~0.9 microns). We also speculate that any
newly discovered VHJs with the right combination of orbital separation and host
star parameters could be thermally detected in the optical. The lack of
detections would still provide constraints on the planetary albedos and
re-radiation factors.

Abstract: The dynamical interactions of planetary systems may be a clue to their formation histories. Therefore, the distribution of these interactions provides important constraints on models of planet formation. We focus on each system’s apsidal motion and proximity to dynamical instability.

Although only ~25 multiple planet systems have been discovered to date, our analyses in these terms have revealed several important features of planetary interactions. 1) Many systems interact such that they are near the boundary between stability and instability. 2) Planets tend to form such that at least one planet’s eccentricity periodically drops to near zero. 3) Mean-motion resonant pairs would be unstable if not for the resonance. 4) Scattering of approximately equal mass planets is unlikely to produce the observed distribution of apsidal behavior. 5) Resonant interactions may be identified through calculating a system’s proximity to instability, regardless of knowledge of angles such as mean longitude and longitude of periastron (e.g. GJ 317 b and c are probably in a 4:1 resonance).

These properties of planetary systems have been identified through calculation of two parameters that describe the interaction. The apsidal interaction can be quantified by determining how close a planet is to an apsidal separatrix (a boundary between qualitatively different types of apsidal oscillations, e.g. libration or circulation of the major axes). The proximity to instability can be measured by comparing the observed orbital elements to an analytic boundary that describes a type of stability known as Hill stability. We have set up a website dedicated to presenting the most up-to-date information on dynamical interactions: this http URL

Comments: 10 pages, 3 figures, 1 table. To appear in the proceedings of IAU Symposium 249: Exoplanets: Detection, Formation and Dynamics, held in Suzhou, China, Oct 22-26 2007. A version with full resolution figures is available at this http URL

Charter

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last seven years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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